Whats wrong with my knee?

my left knee has been feeling really weak lately i went on vacation this past week and while i was walking it felt like my knee gave in or something cuz i could and can still feel something sliding in my knee when i move it a certain way....walking up and down stairs makes it feel even weaker and its been ache as well i'm 15 and i have football starting up in a couple months so i need to know if i should see a doctor about it and what its sounds like to you guys........thanks in advance for any answers